The genesis and dating of the Early Permian granitic rock in the north of Xilin Hot, Inner Mongolia

The Permian granitic rock near the Chaokewula Mountain in the north of Xilin Hot area are very important for the study of the structural and magmatic acitivities in eastern Xing'an-Mongolian orogenic belt. The purpose of this study is to discuss the petrogenesis and tectonic setting based on detailed investigation of petrography,litho-geochemical characteristics and zircon U-Pb chronology of the granitic rock. The rocks are mainly composed of syenogranite,monzonitic granite and orthophyre. Geochemically,the granites are characterized by rich silicon and alkalis as well as depletion of calcium and magnesium,thus belonging to aluminous-peraluminous alkali granite. The rare earth elements demonstrate a slightly high degree of fractionation,and are characterized by richness of LREE and poorness of HREE. The rocks exhibit significant negative Eu anomalies. Trace element analyses show that the rocks are enriched in Rb,Th,K,La,Ce,Nd,Zr,Hf and Sm and depleted in Ba,Ta,Nb,Sr,P and Ti. The zircon saturation temperature of rock is 802℃,with geochemical characteristics of low Sr and high Yb,showing that the rocks were formed in an environment of high temperature and low pressure. Combined with the geological and geochemical data,the authors hold that the rocks belong to aluminous A-type granite,and the sources of rocks were formed in an island-arc environment. The zircon U-Pb age of syenogranite is (275.6±1.3) Ma,and the zircon U-Pb age of orthophyre is (274.2±1.4) Ma,respectively. These results indicate that the rock body was formed in late period of early Permian. With a set of volcanic rocks of Amushan Formation as the research object,the authors obtained the zircon U-Pb age of andesite. Combined with the palaeontology,granitoids and chronological evidence,the authors consider that the Hegenshan Ocean was closed and completed the transformation from collision to post-collision during the period from late Carboniferous to late Early Permian.